In a uni-axial extruding device for plastics materials and the like, it is very important to knead fully a resin, and for the purpose of achieving sufficient kneading, various kneading mechanisms have been proposed in Japan, as illustrated in FIGS. 1, 2 and 3. As is apparent from these figures, particularly the cross-sections of the kneading mechanisms, the spaces through which the resin passes have the same configuration regardless of the angular position of the screw during its rotational motion. Consequently the resin within the screw rotates integrally with the screw and the resin mass undergoes little deformation, so that the kneading mechanisms have the disadvantage that they do not subject the resin mass to a strong shearing stress and thus the kneading effect achieved is small.